docs: kerneldoc-preamble: Test xeCJK.sty before loading

On distros whose texlive packaging is fine-grained, texlive-xecjk
can be installed/removed independently of other texlive packages.
Conditionally loading xeCJK depending only on the existence of the
"Noto Sans CJK SC" font might end up in xelatex error of
"xeCJK.sty not found!".

Improve the situation by testing existence of xeCJK.sty before
loading it.

This is useful on RHEL 9 and its clone distros where texlive-xecjk
doesn't work at the moment due to a missing dependency [1].
"make pdfdocs" for non-CJK contents should work after removing
texlive-xecjk.

Link: [1] https://bugzilla.redhat.com/show_bug.cgi?id=2086254
Fixes: 398f7abd ("docs: pdfdocs: Pull LaTeX preamble part out of conf.py")
Cc: stable@vger.kernel.org # v5.18+
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/c24c2a87-70b2-5342-bcc9-de467940466e@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/kerneldoc-preamble.sty

@@ -70,8 +70,16 @@
 
 % Translations have Asian (CJK) characters which are only displayed if
 % xeCJK is used
+\usepackage{ifthen}
+\newboolean{enablecjk}
+\setboolean{enablecjk}{false}
 \IfFontExistsTF{Noto Sans CJK SC}{
-    % Load xeCJK when CJK font is available
+    \IfFileExists{xeCJK.sty}{
+	\setboolean{enablecjk}{true}
+    }{}
+}{}
+\ifthenelse{\boolean{enablecjk}}{
+    % Load xeCJK when both the Noto Sans CJK font and xeCJK.sty are available.
     \usepackage{xeCJK}
     % Noto CJK fonts don't provide slant shape. [AutoFakeSlant] permits
     % its emulation.
@@ -196,7 +204,7 @@
     % Inactivate CJK after tableofcontents
     \apptocmd{\sphinxtableofcontents}{\kerneldocCJKoff}{}{}
     \xeCJKsetup{CJKspace = true}% For inter-phrase space of Korean TOC
-}{ % No CJK font found
+}{ % Don't enable CJK
     % Custom macros to on/off CJK and switch CJK fonts (Dummy)
     \newcommand{\kerneldocCJKon}{}
     \newcommand{\kerneldocCJKoff}{}
@@ -204,14 +212,16 @@
     %% and ignore the argument (#1) in their definitions, whole contents of
     %% CJK chapters can be ignored.
     \newcommand{\kerneldocBeginSC}[1]{%
-	%% Put a note on missing CJK fonts in place of zh_CN translation.
-	\begin{sphinxadmonition}{note}{Note on missing fonts:}
+	%% Put a note on missing CJK fonts or the xecjk package in place of
+	%% zh_CN translation.
+	\begin{sphinxadmonition}{note}{Note on missing fonts and a package:}
 	    Translations of Simplified Chinese (zh\_CN), Traditional Chinese
 	    (zh\_TW), Korean (ko\_KR), and Japanese (ja\_JP) were skipped
-	    due to the lack of suitable font families.
+	    due to the lack of suitable font families and/or the texlive-xecjk
+	    package.
 
 	    If you want them, please install ``Noto Sans CJK'' font families
-	    by following instructions from
+	    along with the texlive-xecjk package by following instructions from
 	    \sphinxcode{./scripts/sphinx-pre-install}.
 	    Having optional ``Noto Serif CJK'' font families will improve
 	    the looks of those translations.